The present invention relates to a vinyl chloride resin composition having excellent gelation property and transparency, and more particularly to a vinyl chloride resin composition comprising a vinyl chloride resin and a two-stage polymer with a high molecular weight, mainly composed of methyl methacrylate and an alkyl methacrylate with a C.sub.2 -C.sub.8 alkyl group as main components.
Polyvinyl chloride resins have been widely used because of their excellent physical properties and chemical properties. However, they are poor in processability, that is, their processing temperatures are close to their thermal decomposition temperature and moreover it takes a long time to form into a molten state, so it is difficult to process them. Also, when they are subjected to extrusion, the surfaces of the obtained product are bad, when they are subjected to calendering they are insufficiently melt-kneaded, and furthermore, their secondary processability is poor.
In order to improve the above-mentioned defects, many studies have been made and various techniques have been disclosed. For instance, there are methods using a plasticizer, methods wherein the vinyl chloride is copolymerized with another monomer, methods wherein polyvinyl chloride is mixed with another resin (polymer), and the like.
However, according to any of these methods, it is impossible that the processability of the polyvinyl chloride resin can be sufficiently improved while keeping the excellent physical properties and chemical properties that the vinyl chloride resin itself processes originally.
For instance, as to the method using the plasticizer and the copolymerizing method with the other monomer, the physical properties of the obtained product are greatly different from these of the product prepared from the polyvinyl chloride resin alone. Also, as to the mixing method with the other polymer, many of them are techniques wherein the processing temperature of the polyvinyl chloride is lowered by lowering the melt viscosity of the polyvinyl chloride. According to the method, though it appears that the fluidity of the polyvinyl chloride can be improved, the fact is that the polyvinyl chloride is insufficiently mixed with the other polymer since energy is not consumed for mixing the polyvinyl chloride with the other polymer but consumed for letting the polyvinyl chloride flow, thus resulting in insufficient gelation of the polyvinyl chloride. Accordingly, the products obtained therefrom are seemingly transparent but they are inferior in physical properties to products prepared from the polyvinyl chloride sufficiently gelled.
For solving the above-mentioned defects, there is adopted a method wherein a copolymer with a relatively high molecular weight, containing methyl methacrylate as a main component (hereinafter referred to as "agent for improving processability") is added to the polyvinyl chloride, as disclosed in Japanese Examined Patent Publication No. 40-5311. According to the method, the agent for improving processability which is not gelled easily remains as the so-called fish eyes, so the obtained product has the dirty surface.
For improving the defect, Japanese Examined Patent Publication No. 52-49020 and No. 53-2898 disclose that agents for processability are prepared according to a multi-stage copolymerization method. According to the methods, for obtaining the satisfactory effect, a relatively large amount of the agent for improving processability must be added to the polyvinyl chloride. Accordingly, it is required to develop agents for improving processability which can exhibit the desired improving effect by the addition of a small amount.
As discussed in Japanese Examined Patent Publication No. 40-5311, it has hitherto been well known that a copolymer having a high molecular weight and containing methyl methacrylate as the main component is added to the vinyl chloride resin to improve the processability of the polyvinyl chloride resin. It can be easily persumed that the higher the molecular weight of the agent for improving processability, the more excellent the improving effect. However, when adding an agent for improving processability that only its molecular weight is merely increased to the vinyl chloride resin, the obtained product is not always put to practical use. Because, as to such an agent, there is a case that not only the transparency of the product is lowered but also the degree of gelation of the vinyl chloride resin is reduced and the secondary processability, such as elongation at a high temperature, becomes poor.
The cause, though it is not clear in detail how the fine structure of the agent for improving processability in the final composition influences the improvement of the processability of the vinyl chloride resin, can be considered that the dispersibility of the agent for improving processability is lowered by making the molecular weight higher.
An object of the present invention is to provide a polyvinyl chloride resin composition having excellent processability as well as the excellent properties that the vinyl chloride resin itself possesses originally.
This and the other objects of the present invention will become apparent from the following description hereinafter.